Laboratory Rotational Spectroscopy of CaC₃N and CaC₄H

Our understanding of the roles metal-bearing molecules play in interstellar chemistry is limited by the lack of rotational spectroscopic data crucial for astronomical identification/detection. Although calcium is the second most abundant alkaline earth metal in space (after magnesium), only two calcium-bearing molecules have been detected in space, both in the circumstellar envelope of the evolved carbon-rich star IRC+10216. In this work, we report the rotational, fine, and hyperfine structure of two linear calcium-bearing molecules, CaC₃N and CaC₄H, in their ²Σ⁺ ground electronic states. These findings both confirm the hypothesized ionic metal-ligand bonding characteristics of this class of molecules that make them potential candidates for optical cooling applications and point the way to detecting these and other calcium-bearing molecules in carbon-rich stars.